1. Field of Invention
The present invention relates generally to a thermal printer that conveys and prints while pressing the recording medium against a thermal head by means of a platen roller, and relates more particularly to a thermal printer that can prevent foreign matter from adhering and accumulating on the surface of the thermal head in conjunction with conveying the recording medium.
2. Description of Related Art
Thermal heads used in thermal printers generally have heating elements disposed along the printing width on the surface of a ceramic substrate. The ceramic substrate is typically affixed to a metal heat sink and disposed to the printer frame. A platen roller presses against the surface of the thermal head with the recording medium (thermal paper) disposed therebetween. When the platen roller is then turned, the recording medium is conveyed while pressed against the heating elements of the thermal head, and content is printed on the surface of the recording medium.
Paper dust, chaff, and other foreign matter may be on the surface of the recording medium, and this foreign matter may transfer and stick to the surface of the thermal head as the recording medium is conveyed pressed and rubbing against the surface of the thermal head. The color-producing coating on the surface of the recording medium may also rub off and stick to the surface of the thermal head.
When printing to liner-less label paper wound in a roll (that is, label paper that has adhesive applied to the back side and is wound in a roll without a backing liner, similarly to cellophane tape), the adhesive may also transfer and adhere to the front printing side of the paper, and the adhesive may transfer from there to the surface of the thermal head.
Such foreign matter tends to accumulate in an area downstream in the recording medium transportation direction from where the platen roller and thermal head nip the paper (the “nipping area” below). When such foreign matter builds up, the thermal head presses against the surface of the recording medium with the foreign matter therebetween, resulting in insufficient contact with the recording medium, insufficient transfer of heat, and thus print defects such as streaking.
Soiling of the surface of the thermal head is commonly removed by regularly passing a special cleaning sheet between the thermal head and the platen roller. As taught in Japanese Unexamined Patent Appl. Pub. JPA-2004-167751, a polishing function may also be imparted by impregnating the outside surface of the platen roller with an abrasive agent so that foreign matter on the surface of the thermal head is removed by the platen roller rotating while pressed directly against the surface of the thermal head.